1. Field of the Invention
The present invention relates to an incremental rotary encoder which is suitable for surveying instruments such as total stations, theodlites, or the like.
2. Description of the Related Art
Surveying instruments such as total stations, theodlites, or the like, in recent years are provided with an incremental rotary encoder as an angle measuring device for measuring horizontal or vertical angles. This angle is calculated using the sum of a first angle value (counter value) and a second angle value (interpolation value). Two signals of sine and cosine waves (sin .theta., cos .theta.) having a phase difference of 90 degrees therebetween which are output from the sensors of the incremental rotary encoder when it is in operation are each coded into a binary signal with reference to a corresponding reference voltage to obtain the aforementioned first and second angle values. The counter value is obtained by counting the points of variation in the signal level of each of the binary coded signals, while the interpolation value is obtained by dividing a section between two adjacent points of variation in the signal level of the binary coded signal.
The interpolation value is calculated in accordance with the variation of the oscillation from the constant reference voltage. Therefore, if the median voltage of each sine wave signal deviates from the reference voltage thereof, the interpolation value is not precisely calculated, thereby including error.
This error will be hereinafter discussed in detail with reference to FIG. 5. In FIG. 5 it is assumed that Lissajous circle of either the sine or cosine wave that rotates about the intersecting point of the x-axis and the y-axis (each shown by solid lines) of a reference voltage deviates from the intersecting point to rotate about the intersecting point of the x'-axis and the y'-axis (each shown by dotted lines) of a median voltage. In this state, a point "a" on the deviated Lissajous circle has no declination relative to the y'-axis of the median voltage but has a declination ".alpha." relative to the y-axis of the reference voltage. This declination ".alpha." causes an error in an interpolative calculation for calculating the interpolation value.
Since the interpolation value is obtained by dividing an angle corresponding to one count of the aforementioned counter value, the influence of the voltage difference on the error of the interpolation value becomes greater the lesser the number of pulses per revolution of the incremental rotary encoder (i.e., as the angle per one count is greater).